FIG. 1 is a top view of a fingerprint sensor 1, which mainly includes the bezel 2, and a sensing part 3. The bezel 2 provides a strong outer alternating current (AC) signal. The sensing part 3 has control circuits, a sensor array 4 and so on. The sensor array 4 includes M×N sensor cells 5 which work as receiving cells of the AC signal.
FIG. 2 is a sectional view 6 of the fingerprint sensor 1 illustrating its operation. A finger 7 touches the fingerprint sensor 1. An AC signal 8 from the bezel 2 passes through the finger 7 and a protective layer 9, and arrives at a scanning sensor cell 10. All of the sensor cells, including the scanning sensor cell 10 and other sensor cells 11, are on a substrate 12. Each of the sensor cells, 10 and 11, and the portion of the finger surface overlying the respective sensor cell form a capacitor. The capacitance of this capacitor varies depending on whether the respective finger surface is a ridge or a valley. A pattern of capacitor values can be obtained by driving the AC signal 8 through the finger 7 and measuring the variations of these capacitances at each sensor cell. A fingerprint image is thus obtained.
However, there are many disadvantages when the bezel 2 is used as a strong outer AC signal source. First, it increases the material cost of the fingerprint sensor 1 and the complexity of the manufacture processes. Second, it reduces the product yield of the fingerprint sensor. Third, when the whole module is integrated into an electronic device, such as being placed on a cellular phone or similar devices, the bezel 2 makes the industrial design (e.g., structure and outlook) of the device more difficult and may not match the device's mechanical requirement.
One approach to the above problem designates some sensor cells (e.g., sensor cells 11) as transmitting cells and some sensor cells (e.g., sensor cell 10) as receiving cells. A sensor cell either transmits or receives signals, but not both. Such approach suffers from slow frame rates as the cells have to switch between transmit and receive modes. Also, since only some cells transmit signals, such approach may not provide enough transmitting power, therefor lacking required sensitivity for certain fingerprint applications. Furthermore, direct coupling between the receiving cell and nearby transmitting cells can be problematic for such approach.
Accordingly, what is needed is improvement in the structure of the fingerprint sensor and the mechanism to implement the same.